(a) Field of the Invention
The present invention relates to a liquid crystal display, and particularly to a liquid crystal display provided with a compensation film.
(b) Description of the Related Art
A typical liquid crystal display (LCD) includes two upper panels provided with two kinds of electrodes generating an electric field and a liquid crystal layer interposed therebetween. The two kinds of the field-generating electrodes are supplied with electric voltages and the voltage difference therebetween causes electric field. The variation of the electric field changes the orientations of liquid crystal molecules in the liquid crystal layer, and thus it changes the transmittance of light passing through the liquid crystal layer. As a result, the LCD displays desired images by adjusting the voltage difference between the field-generating electrodes.
The liquid crystal material has birefringence that the refractive indices along a molecular long-axis direction and a molecular short-axis direction are different from each other. Due to the birefringence, the refractive index experienced by incident light is varied depending on the viewing direction of an LCD. Therefore, the color characteristic and the light transmittance in an oblique view are different from those in a front view since a linearly-polarized incident light passes through different paths of the liquid crystal layer to have different phases in its polarization.
Therefore, the LCD containing liquid crystal material has several problems such as the change of the contrast ratio depending on the viewing direction, color shift, gray inversion, and etc. In particular, such problems are severe in a twisted-nematic mode LCD, in which liquid crystal molecules are arranged such that their long axes are parallel to the panels while they are twisted spirally along a direction perpendicular to the panels in absence of applied field, and they tend to align parallel to an applied electric field perpendicular to the panels.
It is suggested that at least one retardation film (or phase difference compensation film) is attached to the twisted-nematic mode LCD. The retardation film solves the problem by compensating the phase difference of the light in the several directions.
However, the retardation film does not solve blue shift problem in a black state.